In recent years the advent of modern power electronics has substantially changed the motor and controller hardware used in traction control and electric/hybrid vehicle drive applications. Historically, DC motors have been popular in such applications due to their linear torque response over a wide speed range to input current and their near independent torque and flux control. However, DC motors may exhibit lower efficiency, increased maintenance requirements and speed limitations imposed by the commutator and brushes.
The disadvantages of DC motors can be overcome by the use of synchronous machines in either the form of a permanent magnet (PMAC), wound-field or synchronous reluctance machine (SyR), in conjunction with an inverter. Synchronous machines are robust since they have no brushes or commutators and are capable of four quadrant operation when used with a variable frequency and variable voltage controller, such as a vector control technique. The use of vector control techniques allows the flux producing component and torque producing component of motor current to be decoupled to produce a motor response analogous to that of a DC motor. Vector control techniques of motor control may be computationally intensive and, when first developed, were not practical due to the lack of capability in the digital processing equipment to handle the required calculations. With the rapid advancement of smaller and faster processors, vector control techniques have become practical for control of synchronous machines.
Electric drive systems for an electric vehicle or a hybrid electric vehicle need to have a wide speed range, high torque per ampere, high efficiency, quick dynamic response, and robustness. Synchronous machines are widely considered for such applications due to the advantages mentioned previously. However, the design of such a high performance drive system is challenging because of the motor""s nonlinear characteristics and its parameter variations due to temperature, magnetic saturation, or both. In addition, the variation of drive input values, such as battery voltage, can negatively affect the drive performance and can cause loss of motor control. The present invention allows for improved techniques for vector control and field weakening strategies to optimally control the machine over its entire operating range. Improvement in machine efficiency is an important objective for any electric or hybrid vehicle drive. In an actual electromechanical machine, e.g., motor, the efficiency is determined by various machine parameters, such as resistance and inductance, which are significantly affected by respective operational and/or environmental variables, such as machine temperature and commanded current levels. The machine performance and efficiency is also affected by the level of the voltage supplied to the drive which is determined by the voltage source, inverter and Pulse Width Modulation (PWM) strategy. Thus, the efficiency of a drive system is a complex function of the machine""s characteristics, inverter""s characteristics, and the selected modulation strategy, e.g., PWM. For a given motor and inverter, the control strategy is the primary means of improving the drive efficiency and controllability. By maintaining a high efficiency for the drive system, the energy required is minimized and the cooling system can realize cost savings.
PMAC machines can be designed to posses a significant field-weakened region. This design approach reduces the inverter size, by reducing the required current per phase which reduces the drive unit size and cost. The controller, however, must become more sophisticated in order to properly control the drive to maintain high efficiency and acceptable dynamic performance in the field-weakened range. The most efficient operation for a PMAC above its base speed is achieved when the smallest amount of current is used to weaken the magnetic flux of the magnet. This condition results in operation where the controller operates the machine near the voltage limit of the system. If the available voltage to the inverter is changed or the magnetic flux is varied due to temperature change, the amount of field-weakening current should be adjusted in order to maintain control and high efficiency. The control should also be modified from the traditional approach in order to increase the drive performance and improve stability for the voltage limited condition. The dynamics of the control may be of paramount importance since operation in the voltage limited condition could cause certain undesirable effects, such as voltage saturation, that can cause slow response or loss of machine control.
Synchronous reluctance machines can also be designed to have a significant field-weakened range. Unlike the PMAC, this machine does not have an already established field flux in the form of a magnet so the torque produced is from reluctance torque. In the field-weakened range, the torque and efficiency are limited by the available bus voltage. If the field can be increased without exceeding a voltage limit greater efficiency and higher torque can be produced from the machine at a given operation point. Thus, means for setting the flux as a function of operating conditions is desired. Presently known control strategies are believed to lack such control function.
Thus, there are two important issues in the control of synchronous machines: stability and efficiency. If too little current is used to weaken the field for a PMAC machine or, analogously, if too much flux current is used in a SyR machine, then control can be lost due to voltage limit conditions. In contrast, if too much current is provided in a PMAC machine or, analogously if too little flux current is used in a SyR machine, then the efficiency will be too low.
As suggested above, improvement in the efficiency of the synchronous machine is another very desirable feature in any electric or hybrid electric vehicle propulsion drive system. Unfortunately, the various parameters of even a carefully designed high-efficiency synchronous machine, such as mutual inductance, magnet flux strength (if applicable), etc., may drastically vary as a function of several operational conditions, such as frequency, excitation level, generated torque, rotor temperature, etc. Thus, the efficiency of a drive system and the required voltage to maintain an operating point is a complex function of a myriad of factors, such as the type of machine used, its parameter sensitivity, inverter topology, the selected modulation technique used for generating respective gate signals to trigger the inverter, etc.
Thus, in view of the foregoing considerations, it would be desirable to maintain the optimal efficiency and voltage control of the drive for the various loads where the drive system is designed to operate. The present invention recognizes that such optimal efficiency may be obtained by appropriately selecting the flux value to be used under varying load and/or torque conditions. In particular, it would be desirable to determine, experimentally and/or analytically, a set of optimal flux values under different speed and torque conditions. The experimentally or analytically-derived flux values could then be incorporated into a memory unit, e.g., a look-up table, to improve the overall efficiency of the drive system. It would be further desirable to make the foregoing technique for improving drive system efficiency substantially impervious to variations in battery voltage and operating temperature, parameter variations from machine to machine, as well as inaccuracies in the experimentally and/or analytically derived data. For example, due to such variations, the maximum efficiency may not be obtained at certain operating conditions, and could even lead to loss of control of the system, particularly in the field-weakening region. In order to improve the stability of the drive system, it would be desirable to provide an adaptive optimal efficiency control technique that, even in the presence of wide variation of battery voltage and machine temperature, would allow the propulsion system to provide both high efficiency and stability over the entire range of speed and torque expected to be encountered by the propulsion system.
Generally speaking, the present invention fulfills the foregoing needs by providing in one exemplary embodiment thereof a method for controlling a synchronous machine. The method allows for calculating a stator voltage index. The method further allows for relating the magnitude of the stator voltage index against a threshold voltage value. An offset signal is generated based on the results of the relating step. A respective state of operation of the machine is determined. The offset signal is processed based on the respective state of the machine.
The present invention further fulfills the foregoing needs by providing in another exemplary embodiment thereof a method for controlling a synchronous machine. The method allows for calculating a stator voltage index. The method further allows for relating the magnitude of the stator voltage index against a threshold voltage value. A parameter value that corresponds to a direct component of a stator current command based on the results of the relating step is generated. This parameter value constitutes a first value of the component of stator current command.
Memory configured to supply a respective parameter value corresponding to the stator current command based on one or more operational signals of the machine is provided. This parameter value constitutes a second value of the component of stator current command. A respective state of operation of the machine is determined. A switching signal is generated based on the respective state of the machine. The switching signal is used for controlling which of the first and second parameter values of the component of stator current command is to be used for machine control.
In another aspect of the present invention, a system for controlling a synchronous machine is provided. The system includes a calculating module configured to calculate a stator voltage index. The system further includes a comparator configured to relate the magnitude of the stator voltage index against a threshold voltage value. A generating module is configured to generate an offset signal based on the results of the comparator. A determining module is configured to determine a respective state of operation of the machine. A processor is configured to process the offset signal based on the respective state of the machine.
In yet another aspect of the present invention, a system for controlling a synchronous machine includes a calculating module configured to calculate a stator voltage index. The system further includes a comparator configured to relate the magnitude of the stator voltage index against a threshold voltage value. A generating module is configured to generate a parameter value corresponding to a direct component of a stator current command based on the results of the relating module. This parameter value constitutes a first value of said component of stator current command. Memory is configured to store a respective parameter value corresponding to the stator current command based on one or more operational signals of the machine. This parameter value constitutes a second value of the component of stator current command. A determining module is configured to determine a respective state of operation of the machine. A switching control unit is configured to generate a switching signal based on the respective state of the machine. This switching signal is used for controlling which of the first and second parameter values of the component of stator current command is to be used for machine control.